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In team sports, substitution (or interchange) is replacing one player with another during a match. Substitute players that are not in the starting lineup (also known as bench players, backups, alternates, or reserves) reside on the bench and are available to substitute for a starter. Later in the match, that substitute may be substituted for by another substitute or by a starter who is currently on the bench.
Some sports have restrictions on substituting or interchanging players whereas others do not. American football, ice hockey, and basketball are examples of sports which practice "unlimited" substitution, albeit subject to certain rules. Substitution is unlimited during play in ice hockey. In basketball, substitution is permitted only during stoppages of play, but is otherwise unlimited. In baseball and association football (soccer), substitution is permitted only during stoppages of play, and a player who has been substituted out of a game usually cannot re-enter it.
In motorsports, a substitution behind the wheel goes by the term "relief driver."